The present application relates to an optical film having refractive index anisotropy, a method of manufacturing the same, and a display unit including an optical film having refractive index anisotropy.
In these years, cathode ray tubes (CRT) that were a mainstream of display units in the past have been replaced with liquid crystal displays, since the liquid crystal displays have advantages such as the low electrical power consumption, the space-saving feature, and the low cost.
There are several types of the liquid crystal displays when categorized by, for example, illumination methods in displaying images. Representative examples include a transmissive liquid crystal display that displays images by using a light source arranged behind a liquid crystal panel.
In such a display, it is extremely important to decrease the electrical power consumption and increase the display luminance in order to increase the commercial value of the display. Thus, it has been strongly aspired that the gain of the optical system provided between the liquid crystal panel and the light source is increased, while the electrical power consumption of the light source is kept low as much as possible.
For example, a technique that a prism sheet as a luminance enhancement film is provided between the liquid crystal panel and the light source has been disclosed. In the prism sheet, for example, a plurality of prisms in the shape of an isosceles triangle pole having an apex angle of 90 degrees are arranged in parallel on a resin film. The front luminance is able to be increased by using the light-focusing effect of the prisms. Further, a technique that prisms in which refractive index in the extending direction of the prisms is different from refractive index in the arrangement direction of the prisms are used in the foregoing prism sheet has been disclosed. In such a prism sheet, in addition to the light-focusing effect of the prisms, polarization split is performed on the inclined plane of the prisms by using interfacial reflection due to the difference of critical angles, and thereby the front luminance is able to be increased.
In the prism sheet having the polarization split function as described above may be fabricated as follows. For example, a plurality of columnar prisms are formed on one face of a sheet containing a semicrystalline resin or a crystalline resin. After that, the sheet is stretched in one in-plane direction (see Japanese Unexamined Patent Application Publication No. 01-273002 and U.S. Patent Publication No. 2006/0138702).
However, in the foregoing fabrication method, there has been a disadvantage that the shape of the prisms is easily collapsed in stretching, and accurately obtaining a desired structure is difficult. Further, in the foregoing fabrication method, to keep the original three dimensional shape even after stretching, it is necessary to stretch the sheet in the ridge line direction of the prisms. Further, in a positive resin with high refractive index in the stretching direction, a refractive index difference generated by stretching is generally larger than that of a negative resin with low refractive index in the stretching direction. Thus, to obtain high birefringence, the positive resin is generally used. Therefore, in the high-birefringent prism sheet fabricated by stretching, the refractive index in the ridge line direction is generally larger than the refractive index in the direction orthogonal to the ridge line direction.